First of all, thank you for your courage in sharing your story. Since SPD can't be "cured" all of our SPD kids will grow up to be adults with SPD. Yes, being diagnosed young will help them learn adapting skills and prepare them to live a more "normal" life, but they'll still have SPD. I'm glad you've found this site and are beginning to get some support. After thinking about your posts, I'd like to offer some advice/guidance, which may help you when you're ready.
First, go ahead and contact child psychiatrists--as many as you can--until you find one that will either treat you even though you're an adult or will refer you to one who knows about SPD.
Second, I'd recommend you read the book Growing an In-Sync Child. Yes, child is in the title, but there are really great activities and exercises in this book that you could try that may be great for you. My husband and I do them with our daughter, and there's nothing about them that makes them only for kids.
Third, regarding your seasonal depression, buy a plant grow light. You could spend more than a $100 to buy one of those broad spectrum light bulbs, but for about $10 you can just get a plant grow light bulb from a garden or hardware store, put it in one of the lamps you already have, and sit under it about 20 minutes a day with a sleeveless top so your full arms are exposed. I learned this from a friend who is a psychiatric nurse who treats lots of people with seasonal affective disorder/depression, and this is what she prescribes. Don't do it for more than 20 minutes a day, though.
Lastly, consider some dietary changes. Artificial colors, flavors and preservatives are a major contributor to symptoms of autism, SPD, ADHD and many more. My daughter has been on the Feingold Diet since October, and the improvement in her behavior and SPD symptoms has been dramatic. The biggy for her was BHT, which is in thousands of things Americans buy all the time. If she drinks two ounces of milk that has BHT (used as a preservative in Vitamin A Palmitate--so it's not listed on the label) within an hour, she's screaming, kicking, banging her head on the floor and wall, hitting, weeping, wetting her pants--you get the picture. It takes anywhere from 2 days to a month to detox depending on how much BHT is built up in your system. Check out the site Feingold.org if you want to read other people's stories and more about the diet.
I hope these ideas are helpful. I applaud you for your openness and willingness to work on finding help for this disorder.